The invention relates to a display tube comprising an electron gun system, a longitudinal axis, a display screen and an electromagnetic deflection unit, which unit comprises a line deflection coil system having two line deflection coils facing each other.
In monochrome display tubes the electron gun system is adapted to generate one electron beam, whereas in, for example colour display tubes of the in-line type the electron gun system is adapted to generate three coplanar electron beams which converge on the display screen.
The electromagnetic deflection unit for deflecting electron beams is used for deflecting the electron beams in two orthogonal directions from their normal undeflected straight path so that the beams impinge upon selected pixels of the display screen so as to provide visual indications on this screen. The electron beams can be moved up or down or from left to right across the (vertically arranged) display screen by suitably varying the magnetic deflection fields. A visual presentation of information or a picture can be formed on the display screen by simultaneously varying the intensity of the beams. The deflection unit, which is secured to the neck portion of the display tube, comprises two systems of deflection coils for deflecting the electron beams in two directions which are transverse to each other; a line deflection coil system to which a line deflection signal of a higher frequency is applied during operation and a field deflection coil system to which a field deflection signal of a lower frequency is applied during operation. Each system comprises two coils arranged with respect to the tube axis at positions facing each other.
An annular core of magnetizable material surrounding the systems of deflection coils if both systems are of the saddle type, is generally used for concentrating the deflection fields and for increasing the flux density in the deflection area.
To satisfy given requirements of convergence (and raster) quality, magnetic six-pole field components are generally to be added to the (dynamic) magnetic dipole deflection fields. The effect of a positive six-pole component on the dipole deflection field is a pincushion-field variation. The effect of a negative six-pole component is a barrel-shaped field variation.
A pincushion-shaped field is generated when the two coils of a system of deflection coils have large window apertures, whereas a barrel-shaped field is generated when they have small window apertures. For a self-converging system the line deflection field in the central area must be pincushion-shaped (the separate line deflection coils must thus have a large window aperture), while it must be homogeneous, more pincushion-shaped or less pincushion-shaped at the screen side, dependent on the quantity of admissible raster distortion, and barrel-shaped (i.e. small window aperture) at the gun side. (Such a field variation is also referred to as field modulation.) Similar field modulations are also important for monochrome systems of display tubes and deflection units which must have a high resolving power.
The flatter the display screen (for example, "superflat" display screens), the deeper the field modulations should be to satisfy the convergence and raster requirements.
Until now it has been found impossible to manufacture deflection coils having a window aperture which varies as much as is desired for said applications, while using the conventional winding methods. However, there are different compromise solutions to reduce the required variation. For example, the barrel shape can be increased by means of plates of a soft-magnetic metallic material. The use of metal plates in the deflection field is, however, undesired if the display tube/deflection unit is to be operated at higher frequencies (EVTV, HDTV). In fact, the energy generated by eddy currents in the metal plates cannot be dissipated in a simple manner so that the temperature of the deflection coil(s) may become inadmissibly high.